1. Field of the Invention
The present invention relates to a multilayer zinc oxide (ZnO) varistor and a composition for use in the multilayer ZnO varistor, in particular, to a composition containing at least 80 mole % of ZnO and at most 20 mole % of additives for the multilayer ZnO varistor with variable breakdown voltages.
2. Description of the Prior Art
Generally speaking, a sudden surge of voltage or electric current inevitably occurs in electric or signal circuits. The source of these surges is voltage transients mainly resulting from electro-static discharge (ESD), lightening or, starting up and on/off switching operations of generators or motors. Voltage surges and disturbances can damage electric components of the electric circuits and even cause fire. Varistors, also known as surge absorbers, are normally used to protect electric circuits and electric components against spurious voltage surges and voltage transients.
Technically an electro-static discharge is defined as a transfer of electrostatic charge between two bodies at different electrostatic potentials caused by direct contact or induced by an electrostatic field. When we walk across a floor, a charge (static electricity) builds up in our body due to the friction between our shoe material and the flooring material. If we approach or touch an object with a lower electrostatic potential then the static electricity discharges from our body into the object.
Today, electronic circuits are becoming smaller and more sensitive to external interference like ESD. This invention provides multilayer chip varistor with the better ability to protect components from destructive transient over-voltage of ESD. Since ESD is a transient over-voltage generated by the friction of human body, as electronic systems become more portable, and the transient susceptibility of semiconductors increases, government regulations are essential to maintain a minimum level of performance in all equipment. Europe is so serious about the problem that they require that equipment be certified via testing to meet IEC 61000-4 series ESD test specifications after 1996.
Varistors are resistors with resistance varying with voltage with a nonlinear coefficient. Varistors have high resistance and are good resistors when loaded with voltage below the critical voltage. However, when voltages are higher than the critical voltage, the resistance of the varistor sharply decreases and the electric current through the varistor will greatly increase. That is, varistors possess the ability to adsorb surges, reduce overload voltage to a safe level and prevent electric components from being damaged by surges. Hence, varistors are called “surge absorbers” or “transient over-voltage suppressor”.
The most important electric characteristic of varistors is the breakdown characteristics which can be represented in accordance with the relationship of I=KVα, wherein “I” represents the electric current through the element, “K” is a constant, “V” represents the voltage applied across the element and “α” is a nonlinear coefficient.
The value of “V” selected to give a 1 mA current through the element is called the “breakdown voltage”, abbreviated as “V1 mA”. The greater the value of “α”, the more significant the influence of the voltage on the electric current. In other words, the greater the value of “α”, the better the voltage control characteristic, and the stronger the protecting ability of the varistor will be for the electric circuit.
Zinc oxide varistors, having an extremely high value of nonlinear coefficient and a significantly excellent surge or electric static discharge absorbing capability, are widely applied as surge absorbing elements, arresters, and voltage stabilizer elements, etc. To make compact communication equipment, the trend is to make electric components which are light weight, thin short in length, small in size, having low power consumption, and operating at low voltages. Circuit protecting devices, such as varistors, naturally should also meet the above requirements.
The breakdown voltage of zinc oxide varistors is represented by the equation of V1 mA=Vg·D/d, wherein “D” represents the thickness of the varistor layer between two parallel internal electrodes in a varistor, “d” represents the size of a parallel grain and “Vg” represents the breakdown voltage per grain boundary. The “Vg” value of ZnO varistors is found experimentally to be about 3-4 V which is not influenced by changes in the compositions of additives or manufacture temperatures. Hence, in order to produce zinc oxide varistors with low breakdown voltage, the control of the parameters “D” and “d” are important. The grain sizes of the varistor can be controlled by varying the composition and the sintering temperature. For varistors used at low voltages, grain growth promoters such as TiO2 or seeds are added into the varistor composition to promote the grain growth. However, the addition of the grain growth promoters would result in abnormal grain growth. The distribution of grain sizes is difficult to control and the surge withstanding capabilities of the varistors is reduced. Alternatively, the growth of average grain size can be achieved by increasing the sintering temperature. However, the sintering temperature has an upper limit of about 1,400° C., above which zinc oxide and additives will be evaporated and thereby, the characteristics of the varistor are lost. Hence, the lower limit of the breakdown voltage is also influenced and therefore limited.
For varistors used at low voltages, control of the parameter “D” can be achieved by reducing the varistor thickness by using any of the thin foil method, the sandwich method, the thick film method or the multilayer method.
The thin foil method utilizes conventional plate-pressing machines to produce a varistor with thickness of about 0.3 mm which is the lowest limit in this method. However, the precision of thin foil type varistors is difficult to control and the quality thereof is poor.
The sandwich method relates to addition of additives into zinc oxide single crystal chips and sintered zinc oxide poly-crystal ceramic chips and then heating at high temperatures to produce sandwich type varistors. The additives will diffuse into okay ceramic chip at the high temperature. The breakdown voltage of the produced varistor is quite low; for instance, about 3 V, but the surge withstanding capability thereof is poor for practical use.
The thick film method comprises the steps of forming a slurry of zinc oxide, additives and glass; screen printing heat resistant Pt or Pd conductive gels onto alumina substrate; applying a coating of the slurry of about 100-200 μm thickness thereon; co-firing at high temperature; printing silver gel on the combination obtained above and baking to obtain thick film type varistors. Theoretically, the nonlinear coefficient “α” of thick film varistors is only about one half that of ordinary varistors. That means, at low breakdown voltages, it has a low value of nonlinear coefficient “α” and needs a substrate. Further, because of the comparatively poor compactness, thick film type varistors have poor surge withstanding capability.
Meanwhile, there is an accelerated trend toward smaller electronic apparatus, that is, the spread of mobile communication apparatus, which is accompanied by significant trend toward smaller electronic components and devices of various types. While this has resulted in an increased demand for smaller varistors, a reduction in the size of the varistor leads to a reduction in the effective area of the electrode thereof. For this reason, a need has arisen for varistor elements having improved ESD/surge resisting capability per unit area.
In U.S. Pat. No. 4,290,041, Utsumi et al. utilized a concept in manufacturing multilayer capacitors to produce multilayer zinc oxide varistors with variable breakdown voltages. Because Bi was known to react with many metals in preparing multiplayer ZnO varistors, glass rather than Bi2O3 was used. The Utsumi's method comprises the steps of tape casting green sheet, printing internal electrodes, laminating, cutting, sintering and applying external electrodes. Pb—B—Zn—Si (Borosilicate-lead-zinc) glass is used to replace conventional component Bi2O3 in varistor compositions. The characteristics of varistors are originated in the interface of zinc oxide and glass. In order to retard the degradation, an adequate amount of glass is generally added to manufacture zinc oxide disk-type varistors. This system due to having glass additive would have good life testing result. However, the addition of glass would reduce the surge and electric static discharge withstanding capabilities of the varistors. For this reason, glass is normally not added for disk type varistors in commercial production processes. The addition of Pb—B—Zn—Si glass to substitute Bi2O3 in varistor compositions in Utsumi's patent finally results in poor surge/ESD withstanding capability. On the other hand, the composition of pb in glass is not suit for the trend of environment request.
The present invention provides a bismuth-free ZnO based metal oxide varistor which do not content glass or bismuth but instead contain alkaline earth element Barium as primary additive to improve ESD withstanding capability of the multiplayer varistors. More specifically, the invention provides a varistor with a controllable voltage while improve its ESD withstanding ability.